A tax-preparation glitch affecting about 660,000 tax returns will delay refunds by as long as six weeks, with customers of the nation's largest tax preparer among those affected.

The Internal Revenue Service says the problem involved a form used to claim educational credits and occurred between Feb. 14 and Feb. 22. A statement from the IRS says a "limited number of software company products" were involved.

The IRS doesn't name any companies. But H&R Block has been informing customers about problems. H&R Block spokesman Gene King says the company isn't saying how many of its customers were affected.

The IRS says the number of returns affected is about 10 percent of the total returns claiming the credit. The agency vowed to "aggressively" address the situation.